As was noted in the previous chapter, most plant cells are specialized to a greater or lesser degree, and arranged together in tissues. A tissue can be simple or complex depending upon whether it is composed of one or more than one type of cell. Tissues are further arranged or combined into organs that carry out life functions of the organism. Plant organs include the leaf, stem, root, and reproductive structures. The first three are sometimes called the vegetative organs and are the subject of exploration in this chapter. Reproductive organs will be covered in Chapter 5.
The relationships of the organs within a plant body to each other remains an unsettled subject within plant morphology. The fundamental question is whether these are truly different structures, or just modifications of one basic structure (Eames, 1936; Esau, 1965).
The plant body is an integrated, functional unit, so the division of a plant into organs is largely conceptual, providing a convenient way of approaching plant form and function. A boundary between stem and leaf is particularly difficult to make, so botanists sometimes use the word shoot to refer to the stem and its appendages (Esau, 1965).
The Leaf -The plant leaf is an organ whose shape promotes efficient gathering of light for photosynthesis, but the form of the leaf must also be balanced against the fact that most of the loss of water a plant might suffer is going to occur at its leaves. Leaves are extremely variable in details of size, shape, and adornments like hairs. Although the leaves of most plants carry out the same very basic functions, there is nonetheless an amazing variety of leaf sizes, shapes, margin types, forms of attachment, ornamentation (hairs), and even color.
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Examine the Leaves (forms) page to learn the extensive terminology used to describe this variation. Consider that there are functional reasons for the modifications from a “basic” type. The Stem The stem arises during development of the embryo as part of the hypocotyl-root axis, at the upper end of which are one or more cotyledons and the shoot primordium. The Root The root is the (typically) underground part of the plant axis specialized for both anchoring the plant and absorbing water and minerals. Root (Follow any links for terms you do not understand and to gain a complete picture of root structural variation) Be sure to read about and understand the meaning of each (at a minimum) of the following terms: adventitious roots, endodermis, epidermis, gravitropism, root cap, root hair, stele, taproot. Most of the material you have read discusses the root organ as found in the angiosperms (flowering plants).
However, among the vascular plants, only Psilotales lack such an organ, having instead rhizomes that bear hair-like absorbing structures called rhizoids (Eames, 1936 in Esau, 1965).